1. Field of the Invention
The present invention relates to photography and more particularly to an apparatus and method for color negative evaluation for determining which filter combination should be used in producing a final color balance print.
IDEALLY, A COLOR PRINT COULD BE MADE BY PLACING IT INTO AN ENLARGER AND THEN SIMPLY EXPOSING IT TO A SHEET OF PHOTOSENSITIVE PRINTING PAPER. Unfortunately, such a simple process almost always produces a print which has a color imbalance; for example, the print may have too much blue, or green, or an overall tint or cast of some other color. The imbalances are caused by many different factors and the characteristics of any two color negatives will rarely be the same. One source of the imbalance is in the printing paper itself and this will depend on normal manufacturing variations from one emulsion coating to another, adverse conditions before exposure, illuminating and exposure time, adverse storage conditions between exposure and processing, non-standard processing conditions, and variations in the response of the emulsions of the paper itself. Additional sources of imbalance include variations in the color temperature and age of the enlarger lamp, variations in the internal reflecting color of the enlarger, and the age or freshness of the processing chemicals.
Therefore, to produce a color balanced print, that is, a print closely reproducing the color of the original subject, it is necessary to use filters which are preferably placed above the negative during the printing process.
Following the rule that a color cast is removed by a filter of the same color, a suitable filter can be found to correct for any unnatural color predominance. For example, a pale yellow filter can be used if the print has a slight yellow cast to it; and if the yellow cast is strong, a deep yellow filter can be used. Similarly, a magenta filter can be used to eliminate magenta tints or casts, and cyan filters can be used to control cyan casts. Yellow, magenta, and cyan are the principal filter colors and these colors can be combined for filtering other colors such as blue, red, or green. For example, a green cast can be removed by a cyan and yellow combination, a blue cast by a combination of magenta and cyan, and a red cast by a combination of magenta and yellow. The cast may be yellowish green indicating that its yellow component is stronger than its cyan component so that the yellow filter in combination must be denser than the cyan.
The method of using a series of graduated filters of each of the three principal yellow, magenta, and cyan colors is referred to as subtractive or white light printing. Because paper is made up of three color responding layers, only one or two series of filters are needed to correct a given negative, usually yellow and magenta.
Filters are graduated in terms of density and in the typical filter designation 20 Y, 20 stands for a density of 0.20 and Y for the color yellow which is the filter color. The yellow filter is actually blue light absorptive, and it is the blue light that gives the yellow cast to a print; therefore, a 20 yellow filter placed in a light beam reduces the blue light exposure by 0.20log exposure units. Filter densities can range from 0, that is, no absorption, and upwards; for example, the designation 40 corresponds to a reduction of 0.40log exposure units.